Fix multiple inheritance in `FormattedContent`

See https://github.com/boto/boto/issues/1936

`XMLTemplate` already inherits from `object`, which leads to a `TypeError`:

```
>>> from boto.mturk.connection import MTurkConnection
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/vagrant/debug-boto/local/lib/python2.7/site-packages/boto/mturk/connection.py", line 33, in <module>
    from boto.mturk.question import QuestionForm, ExternalQuestion, HTMLQuestion
  File "/home/vagrant/debug-boto/local/lib/python2.7/site-packages/boto/mturk/question.py", line 182, in <module>
    class FormattedContent(object, XMLTemplate):
TypeError: Error when calling the metaclass bases
    Cannot create a consistent method resolution
order (MRO) for bases object, XMLTemplate
```